1. Field of the Invention
The invention relates to an apparatus for plasma processing of semiconductor wafers, and more particularly, to an electrode assembly wherein the electrode is bonded to a support member. The invention also relates to a process of assembling the electrode and processing of a semiconductor substrate with the electrode assembly.
2. Description of the Related Art
Electrodes used in plasma processing reactors for processing semiconductor substrates such as silicon wafers are disclosed in U.S. Pat. Nos. 5,074,456 and 5,569,356, the disclosures of which are hereby incorporated by reference. The '456 patent discloses an electrode assembly for a parallel plate reactor apparatus wherein the upper electrode is of semiconductor purity and bonded to a support frame by adhesive, solder, or brazing layer. The soldering or brazing layer can be low vapor pressure metals such as indium, silver and alloys thereof and the bonded surfaces of the support frame and the electrode can be coated with a thin layer of metal such as titanium or nickel to promote wetability and adhesion of the bonding layer. It has been found that metallurgical bonds such as In bonds cause the electrode to warp due to differential thermal expansion/contraction of the electrode and the part to which the electrode is bonded. It has also been found that these metallurgical bonds fail at high plasma processing powers due to thermal fatigue and/or melting of the bond.
Dry plasma etching, reactive ion etching, and ion milling techniques were developed in order to overcome numerous limitations associated with chemical etching of semiconductor wafers. Plasma etching, in particular, allows the vertical etch rate to be made much greater than the horizontal etch rate so that the resulting aspect ratio (i.e., the height to width ratio of the resulting notch) of the etched features can be adequately controlled. In fact, plasma etching enables very fine features with high aspect ratios to be formed in films over 1 micrometer in thickness.
During the plasma etching process, a plasma is formed above the masked surface of the wafer by adding large amounts of energy to a gas at relatively low pressure, resulting in ionizing the gas. By adjusting the electrical potential of the substrate to be etched, charged species in the plasma can be directed to impinge substantially normally upon the wafer, wherein materials in the unmasked regions of the wafer are removed.
The etching process can often be made more effective by using gases that are chemically reactive with the material being etched. So called "reactive ion etching" combines the energetic etching effects of the plasma with the chemical etching effect of the gas. However, many chemically active agents have been found to cause excessive electrode wear.
It is desirable to evenly distribute the plasma over the surface of the wafer in order to obtain uniform etching rates over the entire surface of the wafer. For example, U.S. Pat. Nos. 4,595,484, 4,792,378, 4,820,371, 4,960,488 disclose showerhead electrodes for distributing gas through a number of holes in the electrodes. These patents generally describe gas distribution plates having an arrangement of apertures tailored to provide a uniform flow of gas vapors to a semiconductor wafer.
A reactive ion etching system typically consists of an etching chamber with an upper electrode or anode and a lower electrode or cathode positioned therein. The cathode is negatively biased with respect to the anode and the container walls. The wafer to be etched is covered by a suitable mask and placed directly on the cathode. A chemically reactive gas such as CF.sub.4, CHF.sub.3, CClF.sub.3 and SF.sub.6 or mixtures thereof with O.sub.2, N.sub.2, He or Ar is introduced into the etching chamber and maintained at a pressure which is typically in the millitorr range. The upper electrode is provided with gas holes which permit the gas to be uniformly dispersed through the electrode into the chamber. The electric field established between the anode and the cathode will dissociate the reactive gas forming a plasma. The surface of the wafer is etched by chemical interaction with the active ions and by momentum transfer of the ions striking the surface of the wafer. The electric field created by the electrodes will attract the ions to the cathode, causing the ions to strike the surface in a predominantly vertical direction so that the process produces well-defined vertically etched side walls.
A showerhead electrode 10 in an assembly for a single wafer etcher is shown in FIG. 1. Such a showerhead electrode 10 is typically used with an electrostatic chuck having a flat bottom electrode on which a wafer is supported spaced 1 to 2 cm below the electrode 10. Such chucking arrangements provide temperature control of the wafer by supplying backside He pressure which controls the rate of heat transfer between the wafer and the chuck.
The electrode assembly is a consumable part which must be replaced periodically. Because the electrode assembly is attached to a temperature-controlled member, for ease of replacement, it has been conventional to metallurgically bond the upper surface of the outer edge of the silicon electrode 10 to a graphite support ring 12 with indium which has a melting point of about 156.degree. C. Such a low melting point limits the amount of RF power which can be applied to the electrode since the RF power absorbed by the plasma causes the electrode to heat up. The electrode 10 is a planar disk having uniform thickness from center to edge thereof. An outer flange on ring 12 is clamped by an aluminum clamping ring 16 to an aluminum temperature-controlled member 14 having water cooling channels 13. Water is circulated in the cooling channels 13 by water inlet/outlet connections 13a. A plasma confinement ring 17 comprised of a stack of spaced-apart quartz rings surrounds the outer periphery of electrode 10. The plasma confinement ring 17 is bolted to a dielectric annular ring 18 which in turn is bolted to a dielectric housing 18a. The purpose and function of confinement ring 17 is to cause a pressure differential in the reactor and increase the electrical resistance between the reaction chamber walls and the plasma thereby confining the plasma between the upper and lower electrodes. A radially inwardly extending flange of clamping ring 16 engages the outer flange of graphite support ring 12. Thus, no clamping pressure is applied directly against the exposed surface of electrode 10.
Process gas from a gas supply is supplied to electrode 10 through a central hole 20 in the temperature-controlled member 14. The gas then is distributed through one or more vertically spaced apart baffle plates 22 and passes through gas distribution holes (not shown) in the electrode 10 to evenly disperse the process gas into reaction chamber 24. In order to provide enhanced heat conduction from electrode 10 to temperature-controlled member 14, process gas can be supplied to fill open spaces between opposed surfaces of temperature-controlled member 14 and support ring 12. In addition, gas passage 27 connected to a gas passage (not shown) in the annular ring 18 or confinement ring 17 allows pressure to be monitored in the reaction chamber 24. To maintain process gas under pressure between temperature-controlled member 14 and support ring 12, a first O-ring seal 28 is provided between an inner surface of support ring 12 and an opposed surface of temperature-controlled member 14 and a second O-ring seal 29 is provided between an outer part of an upper surface of support ring 12 and an opposed surface of member 14. In order to maintain the vacuum environment in chamber 24, additional O-rings 30, 32 are provided between temperature-controlled member 14 and cylindrical member 18b and between cylindrical member 18b and housing 18a.
The process of bonding the silicon electrode 10 to the support ring 12 requires heating of the electrode to a bonding temperature which may cause bowing or cracking of the electrode due to the different thermal coefficients of expansion of the silicon electrode 10 and the graphite ring 12. Also, contamination of wafers could result from solder particles or vaporized solder contaminants deriving from the joint between electrode 10 and ring 12 or from the ring itself. During high-power plasma processing, the temperature of the electrode may even become high enough to melt the solder and cause part or all of the electrode 10 to separate from the ring 12. However, even if the electrode 10 becomes partly separated from ring 12, local variations in electrical and thermal power transmission between ring 12 and electrode 10 could result in nonuniform plasma density beneath the electrode 10.